Over millions of years, subterranean earth formations, such as hydrocarbon reservoirs, undergo geological changes that result in unexpected pressure and rock strength variations. Complex, deep-water and unconventional hydrocarbon reservoirs often exhibit wellbore mechanics that prevent the use of conventional drilling techniques. Managed pressure drilling (MPD) was developed as a group of technologies to more precisely control the annular pressure profile throughout wellbores by creating only a minimal overbalanced annular pressure.
Generally, the formation pore pressure and the fracture pressure increase with the true vertical depth (TVD) of a well. In MPD, for each drilling interval, drilling fluid (i.e., “mud”) may be used that exhibits a pressure that is greater than the pore pressure (Ppore), but less than the fracture pressure (Pfrac), such that a drilling fluid pressure in the annulus lies between the pore pressure and the fracture pressure. The difference, also called window, between downhole Ppore and Pfrac is sufficient so that the equivalent circulating density (ECD) of the drilling fluid remains within the allowable density window. MPD allows operators to keep the ECD within the narrow Ppore/Pfrac pressure window while still maintaining pressures conducive to wellbore stability.